A transmission line is used for high-speed digital signal transmission. For example, differential transmission is generally used for transmitting digital signal with data rate of hundreds of Mbps or higher, and a transmission line for performing the differential transmission is a differential transmission line. Conventional differential transmission line on a planar structure, such as PCB, is formed by a pair of (two) transmission line conductors constructing in a strip shape, and formed in the same layer. One of the conductors carries a positive voltage, and the other conductor carries a negative voltage. The pair of transmission line conductors are generally formed to be parallel to each other and to extend in a straight line shape. The differential transmission is advantageous in that the amplitude of a signal voltage is made to be small, and thus influence of noise hardly exerts.
Generally, high-speed transmission of the differential signal requires the difference in transmission time between the signals (referred to as a skew hereinafter) to be minimal. It is necessary to greatly reduce a skew between the differential transmission lines in order to achieve high-speed digital signal transmission on a low voltage differential signaling system. Nowadays in many applications, differential transmission lines are used as medium for high-speed data transmission between at least two devices. These devices are usually placed in the same outer layer (top or bottom layer) of a PCB, such as the transmitter (Tx) IC 101′ and the receiver (Rx) IC 102′ placed at the same side of the PCB, as shown in FIG. 1. However, direct linkage on the same PCB outer layer between the devices sometimes cannot be established due to the fact that polarity of the terminals 103′, 104′, and 105′, 106′ on the devices do not match to each other. In other words, the polarity is reversed. To complete the linkage, either one of the conductors 112′ or 113′ of the differential transmission lines has to go into the inner layer (or the other outer layer) of the PCB, and return to the original layer again, by adding at least two vias 114′, while the other conductor will need to stay at original outer layer. Based on such arrangement, great skew effect will be generated between the two transmission line conductors to degrade the signal quality as the extra reflection and insertion loss are introduced, thus, a demand for reduction of the skew is needed.
For these reasons, it is desirable to provide an improved differential transmission line structure to overcome the drawbacks.